Because of our constitution and form of government. Some powers are delegated to the federal government, while others are reserved to the states. Again, there can be overlap, but in many cases, like this one, it's a state/local issue, and the responsiblity of the state government to decide how to handle it. Remember that in many ways, NOT passing a law is a decision as well. The federal government should not act as a nanny or parent for the state governments. As horrific as the practice is, it's up to the states to address it.
I refer you to another post I made in this thread...or rather, I will summarize for you so I and you don't have to go hunting for the number: laws are subservient to moral truth. Always have been, always will be. Everyone has a right to resist immoral laws, and conform the law to known moral truth, and to act outside the law when moral truth is at stake.
Simple question: is it morally right, or not, to subject a female child to having her clitoris cut off?
I say the question has a simple answer: Hell. No. It is not morally right.
We have laws because of moral truth. We have laws against murder, for example,
because it is immoral to murder human beings. We have laws against theft, against lying in sworn testimony or in certain other situations, against rape, against child abuse, against poisoning the water supply, against welching on a contract, etc. etc.
because it is immoral to do those things.
When the laws permit something like FGM, the laws be damned, whoever can act, must act. It is a moral imperative that they do, and moral truth supersedes any and all human laws.
Every time in human history a movement of people opposed a law on moral grounds, we have judged them to have acted rightly. The Peasant's Revolt of 1524-5 was in the right, whatever the laws of the realm said at that time. The civil rights activists who broke a great many laws opposing Jim Crow in the South were in the right. The people who ran the underground railroad in the South before emancipation were in the right. Those Germans who concealed and smuggled Jews out of Nazi Germany were in the right. The women in England and the United States who broke laws to gain women's suffrage and basic rights for women were in the right. No one now looks back at those events and says "dammit, those people broke the law, they were horrible people." The reason no one does is because everyone who isn't a psychopath knows that moral truth supersedes mere law.
This is a simple case. FGM is morally repugnant. People who object "but the law! the law!" are in the wrong, as they have been every other time throughout history. We must always be on the side of what is moral, when what is moral is known. And in this case, it absolutely is.